The invention relates generally to gas turbines and more specifically to gas turbine combustors with secondary fuel nozzles.
Gas turbine manufacturers continue research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is an exponential function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will not be produced.
One preferred method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not formed.
Lean, premixing fuel injectors for emissions abatement are in common use throughout the industry, having been reduced to practice in heavy duty industrial gas turbines for more than two decades. Such devices have achieved great progress in the area of gas turbine exhaust emissions abatement. Reduction of oxides of nitrogen, NOx, emissions by an order of magnitude or more relative to the diffusion flame burners of prior art have been achieved without the use of diluent injection such as steam or water.
A common configuration for combustors in gas turbines provides an annular array of primary nozzles each of which discharges fuel into the primary combustion chamber, and a central secondary nozzle which discharges fuel into the secondary combustion chamber. The secondary nozzle has an axial fuel delivery pipe surrounded at its discharge end by an air swirler which provides combustion air to the fuel nozzle discharge. Often the secondary nozzle is operated as a two-stage (diffusion and premixing) gas only secondary fuel nozzle with two fuel circuits. This allows the nozzle to operate in a premixed mode or diffusion mode. The secondary nozzle of each combustor is located within a center body and extends through a liner provided with a swirler through which combustion air is introduced for mixing with fuel from the secondary nozzle. The secondary nozzle is arranged to discharge fuel into a throat region between an upstream primary combustion chamber and a downstream secondary combustion chamber. Fuel is supplied to the secondary nozzle through concentrically arranged diffusion and premix pipes.
FIG. 1 illustrates a premixing section of a prior art secondary fuel nozzle assembly 5. The premixing section 10 includes multiple pegs 15 fixed to an outer surface 16 of outer wall 17 of the fuel nozzle body 20, each with a fillet weld 18. The multiple pegs 15 extend radially outward from the fuel nozzle body and around the circumference at discrete locations. Radially internal to the fuel nozzle wall 17 are multiple secondary manifolds 25, each manifold disposed between an inner surface 19 of the fuel nozzle wall 17 and a support structure 22 radially inward. Also radially inward from the secondary manifolds 25 are fuel chambers 30, which may be supplied with fuel from a rear portion (not shown) of the secondary fuel nozzle assembly. The secondary manifold 25 may include radial passages 26 from the fuel chamber 30 below, communicating through the fuel nozzle wall 17 and through the peg 15. The radial passages communicate with discharge passages 27 and through fuel injection holes 28 into the premixing space 40 around the fuel nozzle body 20. The pegs 15 interrupt the distribution of airflow 45 and result in uneven radial and circumferential mixing of fuel and air. Further, due to the size and their far reach into the premixing space 40, the pegs 15 cause an undesired pressure drop.
Accordingly, there is a need to provide a premixing arrangement for the secondary fuel nozzle that is simple and exercises improved control over fuel-air mixing.